Intracellular C1- activity (aiC1) will be measured in dog cardiac Purkinje fibers and cat papillary muscle under a variety of conditions. These conditions include C1-substitution and application of drugs. The measurements will be made with C1- liquid ion exchanger microelectrodes. The object of the study is to understand the mechanism of C1- transport in heart muscle and its relationship to the electrical activity of the heart. Intracellular K ion and Na ion activities (aiK & aiNa) will be measured in heart muscle preparations in the presence of cardiac glycosides. The object is to obtain information concerning the effect of cardiac glycosides on electrolyte distribution and the relationship of that distribution to the electrical properties of heart muscle.